"An" is used before words that start with a vowel sound, not necessarily a vowel. If the word is an acronym, a number, or a single letter, that is pronounced starting with a vowel, then "an" is used - otherwise "a" is used.
The letter "n" is pronounced: en
So the correct term is [an "n"], not [a "n"].
Aren't you the one supposed to know English grammar, since it's your first language, but Liggy's and my second or third language?! :bigsmile: